Automatic gain control



July 18, 1950 J. ci. coE

AUTOMATIC GAIN CONTROL Filed Nov. 26, 1945 l I9 f3 lf AGG.

le Zo la l 27 'Il 2:1/I

ral l. M 53 35 e JAMES C 00E Patented July 18, 1950 UNITED STATES rgTsNTOFFICE (Granted under the act of March 3, 1883, as amended April 30,1928; 370 0. G. 757) 11 Claims.

This invention relates to differential time .constant automaticgaincontrols for radio receivers, and particularly to automatic gain controlcircuits wherein a rectilier'is provided to develop a gain reducingpotential in a circuit of relatively long time constant, kand anadditional rectier is provided' in a relatively short time constantcircuit to discharge the gain, reducing potential.

A generalv object of the. invention is to provide an automatic gaincontrol (AGC) circuit for a receiver which will tend to maintain highreceiver sensitivity Linder vchanging signal strengths. Thus, theobjective is to cause slow desensitization of the receiver withincreasing signal strength to permithigh amplication of the signal, tocontinue for an appreciable time after the signal strength has startedYto rise, and to cause rapid increase or the amplication as the signalstrength starts to fade.

More particularly; an object of the invention is to provide an AGC'circuitfor a, radio receiver, preferably for amplitude modulated carrierwave reception, which will operate without interfering with thereproduction of the lower frequency audio modulations andat the sametime will Aprovide rapid, recoveryI of the receiver gain upon a'reduction in the average carrier intensity. i

The AGC circuits generally utilized in radio reception employ arectifier for producingv a negative potential which. increasesnegatively with an increasing intensity ofthe incoming carrier. Thispotential isl applied to a filtering network and isv used to control thegain of signal amplifying circuits. The charging time constant of theilterfingl network must be long, enough to insure that low audiofrequencies will not actuate the circuit,

but a short discharge time constant is desirable to permit rapidrecoveryI of sensitivity with signal fading` In theA common AGCcircuits, the charging time will b e appreciably shorter than the dis-vcharge time, as is later explained. The circuit constants chosen for thecommon AGC circuit must be a compromise, and the resultant time constantis usually between '.1 and' 0.2 second.

In a circuit withy a discharge time constant of the order of 0.1secondor longer, atmospheric static and man-madenoise of an intermittentnature may actuate the AGC system resul-ting in receiverdesensitization. The desensitization. will. continue until some timeafter the end ofthe noise; andwhen additional' noise pulses are. re-

ceived, spaced as close together as the discharge time constant of theAGCY system, the system receives each added noise pulse beforetheefects.

of the rst can wearv off; resulting in an accumulated negative AGCvvoltage andA continuing desensitization of thereceiver.-

It is accordingly-an additional object of the invention to provide an-AGC circuitwhich will' have a-` long timel constant. charging circuityto` minimize the desensitizingk effects of a single noise pulse, or ofa series of pulses, but will have a comparatively fast discharging timeto prevent accumulation of AGC voltage from a group of separate pulsesand to restore receiver sensitivity rapidly aiter the end of the, noise.j

Further objects and advantages ofthe invention will appear from thefollowingdiscussion taken in conjunction with the drawings, in which: l

Fig. 1 is a diagrammatic presentation of an automatic gain controlcircuit in accordance with the invention, and

Fig. 2 is a diagrammatic presentation of an automaticfgain controlcircuitv in accordance with the inventionv adapted for incorporationinto the diode detector stage ofa radio receiver circuit.

In accomplishing the objects of the invention, two circuits are. used,each of'whi'ch produces a separate negative biasing` potentialproportioned to the signal intensity. Thel first circuit has, 'a longtime constant on charging, in response to carrierintensity increase, sothat the negative potential produced by this circuit builds up slowlywith increasing carrier intensity. 'Ihe second circuit has a short timeconstant, the condenser of which discharges rapidly with decreasing:signal' intensity-- A rectier, which conducts direct current in onedirection only, prevents` the coni-- denser in the first circuit fromever becoming.

more negative than the condenser in the second.-v

circuit.

The invention, as shown in Fig. 1, is embodied in a circuit comprisingtwo similar diodes I0. andy II, which may be sections of a single tube,such primary winding of the iinal- I. Fi transformer' IE on which the I.F. potential is impressed. The

detector circuit of thev receiver isnot shown but Associated withdiodes'I0# and IIl are iilter-circuits comprisingresistors and capacitors, thevcir-f cuit associated with diode I 0i having afaster time constant thanthat associated-'with diode I I Diode II) is connected to rectifythesignalvoltages' impressed through condenser |14" and to developv`racross load resistor I8 a negative direct current potential of a valuerelated tol the average sig-y nalr strength.. Audio frequencymodulation'of the signal` willv produce audio potentials across the loadresistor, but these will be' subjectedI to ther blocking action of aresistor 211l and Icy-passing through condenser 22; The circuitof'resistor 2U.-

and.y condenser 22ehas.. arelativel-y. fastA timev con--` stant, having;low resistanceiand! capacity,` values-,1:

The connection to: condensersf I4 and I5 is preferably made to that endof'th'eA and a direct current potential produced on the bias potentialis applied to cathode 26 of diode I from a potential source such asbattery 24 t0 delay rectification by the diode until a predeterminedsignal intensity is exceeded.

The signal fed through condenser I5 is applied to the plate I3 of diodeII and is rectified in a manner similar to that described above toproduce across load resistor I3 a D. C. potential and audio frequencypotentials in accordance with the signal. Values for resistor 2| andcondenser 23 are chosen such that their RC product (time constant) isgreater than for resistor and condenser 22. The Vtime constants may bein the ratio of approximately 5:l,but this value is not critical.Positive bias is applied to cathode 21 of diode II by a source indicatedas a battery to delay rectification until a predetermined signalintensity is exceeded, this bias being preferably of substantially equalvoltage and applied as in the circuit of diode I0.

Diode I1 interconnects the ungrounded side of condenser 22 and theungrounded side of condenser 23, cathode 29 being connected directly tocondenser 23, and anode 28 being connected to condenser 22.

In general, the values of circuit constants for satisfactory operationof the device shown in Fig. 1 are selected so that: the product of thevalues of condenser 22 and the series resistance of resistors I8'and 20is less than the product of the values of condenser 23 and the seriesresistance of resistors I 8 and ZI; andthe value of resistor I8 isgreater than the value of resistor I9, or the voltage of battery 25 ismade greater than the voltage of battery 24. Specific values that havebeen found suitable in a particular application are as follows, it beingunderstood that these values 'are exemplary only:

vThe general operation of the device in accordance with Fig. 1 is suchthat diode I1 is normally non-conductive, and the negative potentialproduced on the ungrounded side of condenser 23- by received carrierrectification is available for gain reduction in the r-eceiver, such asfor grid biasing in the I. F. amplifier circuits. 'I'he potentialproduced on condenser 23 is slightly less for a given carrier strengththan that produced on condenser 22, since load resistor I8 is of ahigher value than resistor I94 if bias batteries 24 and 25 vare of equalvoltage. A similar eiect may be obtained by'other circuit adjustments,as by increasing the positive bias voltage on cathode 21v to a greatervalue than for cathode 26. Decreasing carrier intensity results in arapid decrease in the negative potential appearing on the ungroundedsideof condenser 22 and anode 28 of diode I1. Diode I1 is accordingly madeconductive to drain off the negative potential on the ungrounded side ofcondenser 23, reducing the negative AGC bias and increasing receivergain much'more rapidly than would occur if condenser 23 had only thedischarge path through resistors 2| and I9 in series. A subsequentincrease in carrier strength rapidly causes a relatively high negativepotential on condenser 22 and anode 28, which does not reach the AGCoutput, being blocked by the diode I1. The potential on condenser 23 isslowly built up through high resistance 2| resulting in a slow decreasein receiver gain.

In the conventional AGC circuit the gain-reducing potential would beproduced on a condenser, such as condenser 23 through rectication of thecarrier. Charging of the condenser through a series resistor, such asresistor 2l. would be rapid in comparison to the slow discharge throughthe series resistor and the load resistor, such as resistors 2I and I0,in series. As heretofore explained, the circuit of this inventionprovides a substantially slower charging rate than the discharging rateof the AGC potential on condenser 23.

The circuit disclosed in Fig. 2 is in most respects substantially thesameas that of Fig. l. Diode I0 of Fig. 2 is utilized as the detectorfor the receiver to which the device is applied and also furnishes a D.C. potential to condenser 22 and Vanode 28 of diode I1. An R. F. by-passor load-capacitor 30 is provided inparallel with load resistor I8. Theaudio voltages may be taken off in the conventional manner from theanode end of the load resistor as indicated. A resistor 32 may beprovided as shown across condenser 22 to decrease the discharge timelconstant of the condenser 22 if the discharge timethrough resistors 20and I8 is longer than desired. The load resistance I8 is connecteddirectly to the cathode 2B of diode I0 to provide for detection of:signals of low intensity, since the circuit of Fig. 1 would produce nopotentials across the load resistance I8 until the signal reaches anintensity sufiicient to overcome the bias against diod-e I0. Theconnections shown in Fig. 2 cause a positive potential to appear ontheungrounded side of condenser 22 in the absence of a signal of at least apredetermined intensity. This potential would appear, through diodeI1,on the ungrounded sider of AGC condenser- 23 unless ^preventive meanswere provided. Accordingly, diode 3| is connected across the terminalsof condenser 23 in such a way as to cause any positive charge on theungrounded side to be shunted to ground by driving anode 33 positivewith respect to grounded cathode 35, causing conduction of the diode.Connections to the signal rectifying diodes are slightly altered byconnecting anode I2 directly to the I. F; transformer I 6 secondary andfurnishing a condenser I5 between the transformer secondary and anode I3of diode II, although other convenient means of applying signalpotentials tothe diodes will be readily apparent.

Exemplary values for the components of an operative circuit as shown inFig. 2 may be as follows:

condensav l5 -mfd-; .00025 Condenser 22 mid-- .2 Condenser 23 mfd-- .2Condenser 30 mfd .0001 Resistor I8 meg-- 1.0 Resistor I9 meg 0.5Resistor 20 meg 0.1 Resistor 2l r r meg 2.0 Resistor 32 meg 1.0 Battery24 volts 3 Battery .,25,, do 13 Thev operationv of4 the circuitgof Eig.2 i-Sfalso.

basically similar to that of Fig. l.. ,Acarrierfioff sufficientIintensity to overcome, the bias `against diode lil; willfslowly'buildupa negative,- AGCI po-` tential through-` high resistance 21|; on;condenser'. This carrierI will; also; .rapidly produce` a, slightlygreater negative potential; lon,condc-inserl 22 through relatively; lowresistance 2|). =Tl i,e,y tential; on condenser-22.willgbe greaterbecause load; resistor |18.- is l of a; higher value, thanload re.-YThe. negativer potentialY Oni condenser* 23: isappliedV as: anAGC.'potential-to reduce ain-,- plication. intheappropriate;receiver;circuits. A:-

sister I9..

reduction in carrier strength, or fading, will result' in rapid,discharge of condenser 22 through resistors 2|l and 8 in series andthrough the parallel path comprising resistor 32if resistor 32 isincludedin thecircuit. Condenser 2,3. begins to slowly discharge throughresistors .2| and |9 in` vented from appearing on condenser 23 byfdiode3| which isimade conductivetofprevent a positive potential fromappearing on the condenser.

It will be apparent to those skilled in the art that various kinds ofrectifiers, such as metal oxide types, may be substituted for the diodesshown. Alternatively, grid controlled electron tubes may be utilizedinstead of or combined with rectiers. In some applications filters mayprove desirable, as for instance in series with the connection betweenthe ungrounded side of condenser 22 and anode 2B of diode in the circuitof Fig, 1 or Fig. 2. The carrier wave may be applied for rectificationotherwise than from the last I. F. transformer. It may even be desirableunder certain circumstances to obtain one of the voltages needed for thedevice from the cathode resistor of an amplifier tube of the receiver.It is accordingly desired that I be limited only by the scope of theappended claims.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. In a rectifier circuit for alternating current, a condenser connectedfor charging through an alternating current rectier, a relatively highresistance discharge path connected to said condenser, a separate anddistinct relatively low resistance discharge path connected to saidcondenser through a second rectier, and means responsive to a decreasein the current strength and coupled to the second rectier for renderingsaid second rectier conductive.

2. In a rectiiier circuit for alternating current, direct currentpotential storage means, means connected to said storage means forrelatively slowly charging said storage means to a potential related tothe average intensity of the alternating current potential, a separateand relatively rapid discharge means connected to said storage means andcomprising a rectifier in a, fast time ceiver comprising a rectiiier forcharging a ilrsil7 condenser in arelatively long dischargetimecon.-vstant circuit with a gain control potential related,r to the.averagesignal intensity, means for providing a second potential relatedto the average sig-l riall intensityon a second condenser in a relar.tively` fast discharge time, constant circui1t and, rectifier meansconnected between said; condens@` ers forvpreventingthefpotential ofthe. charge onY i l said rstcondenser fromexceeding thc-potential;

of the charge on said. second. condenser.

4; In an automaticy gain control for aradiore--, ceiver, a. short timeconstant circuit including-,a rectiervfor'producing on acondenser apotential related to the average intensity of high-frequen.

cy currentin the receiver, a second rectifier con--v nected tofalongtime constant-circuit for. prodllr.. ing on a second, condenser avsecondpotential.reer lated to thev average intensity of said high fre.-

, quencycurrent, and rectifying means` interconnecting said. circuitsfor rapidly discharging said secondcondenser in response to adifferential po-.. tential` of predetermined polarity between. said.condensers. Y. y 5. In an automatic gaincontrol for aradiojre-f iceiver,v ashorttime constant circuit, including,.a.T

rectifier for producing on. a condenser a poten-A tial relatedA totheqaverageintensity.or high tren quency current in the receiver,asecond rectifier; connected in a long time constant circuit forproducing on a second condenser a second potential related to theaverage intensity of said high frequency current, said second potentialbeing supplied as an automatic gain control voltage, and rectifyingmeans connected between said circuits and responsive to a decrease insaid average intensity for rapidly discharging said second condenser inresponse to rapid discharge of said first condenser.

6. The combination with the detector of a radio receiver of a short timeconstant circuit for producing on a condenser a potential related to theaverage intensity of high frequency current through the detector, arectifier for producing a gain control potential on a second condenserin a long time constant circuit, and rectifier means connected betweensaid condensers and providing a rapid discharge path for said secondcondenser in comparison with its charging rate, said means beingresponsive to a differential potential of predetermined polarity betweensaid condensers.

7. In an automatic gain control for a radio receiver comprising arectier for producing on a condenser in a relatively long charge timeconstant circuit a gain control potential governed by the averagecarrier wave intensity, means for biasing said rectifier to aninoperative condition for signal intensity less than a predeterminedintensity, means for providing a second potential related to the averagecarrier wave intensity on a second condenser in a relatively fastdischarge time constant circuit, means for biasing said last means toinoperative condition for signal intensity less than a secondpredetermined intensity, and discharge means connected between saidcondensers for preventing said gain control potential from exceedingsaid second potential.

8. In a rectier circuit for converting alternating current into directcurrent, a condenser, rst rectifier means for charging said condenser 1to a rst potential related to the potential of th'e'alternating current,a relatively highresistancedischarge' path for said condenser permittingsaid iirst potential to follow only slow variations-of the amplitude ofsaid alternating current, a second condenser, second rectier means forcharging said second condenser to a second potential related to thepotential of saidalternatng current and greater than'said firstpotential, a relatively W resistance discharge path for said secondcondenser whereby said second potential follows rapid variations in theyamplitude of said alternating current and may become less than said rstpotential, and additional rectier means for discharging said rst;condenser through said low resistance discharge path when said secondpotential becomes less than said first potential.

v97,' The combination according to claim 8 wherein `said first rectifiermeans provides a negative potential on one plate of the first-mentionedcondenser, and additional rectier means shunting Said rst-mentionedcondenser for short-circuiting a positive charge on said one plate.

10. The combination in an automatic gain control system for a radioreceiver of a condenser, means for charging said condenser to apotential related to the average intensity of the received carrier, arelatively long time constant discharge path for said condenser, arelatively short 'time constant discharge path connected to saidcondenser through a rectier, the last-mentioned path being separate fromsaid charging means andthe first-mentioned path, and means responsive toa decline inthe carrier1 strength and coupled to said rectier forrendering said .rectifier conductive.

11. In a radio receiver, the combination comprising automatic gaincontrol means including a condenser and means for' relatively slowlycharging said condenser to a potential related to the average intensityof the received carrier, discharge means for said condenser comprising arelatively short time constant circuit separate from said charging meansand connected to said condenser through an electron tube, and meansresponsive to a decreasing received carrier intensity for rendering saidtube conductive.

JAMES C. COE.

REFERENCES CITED The following referencesare of'y record in :the le ofthis patent:

UNITED STATES f PATENTS Number Name Date 1,931,866 Heising Oct. 24, 19332,115,876 RObertS May 3, 1938 2,129,029 Roberts Sept. 6, 1938 2,154,091Holst Apr. 11, 1939 2,156,846 Getaz May 2, .i939 2,252,066 Dallos Aug.12, 1941 2,262,841 Goddard .Nov. 18, 1941 FOREIGN PATENTS Number CountryI Date 673,988 Germany Apr. 1, 1939

